Minimal Pub/Sub with namespace and remapping topics

#python   #ros2  

---

Table of Content

LAB

• Create minimal Pub/Sub with different topic
• remapping topic from command line
• remapping from launch
• add namespace
• change node name

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Minimal nodes

minimal_pub.py

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

TOPIC = "simple"
PERIOD = 1

class MyNode(Node):
    def __init__(self):
        node_name="minimal_pub"
        super().__init__(node_name)
        self.__pub = self.create_publisher(String, TOPIC, 10)
        self.__timer = self.create_timer(PERIOD, self.__timer_handler)
        self.__timer
        self.__counter = 0
        self.get_logger().info("run simple pub")

    def __timer_handler(self):
        self.__counter += 1
        msg = String(data="pub counter: {}".format(self.__counter))
        self.__pub.publish(msg)

def main(args=None):
    rclpy.init(args=args)
    node = MyNode()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

minimal_sub.py

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

TOPIC = "simple1"

class MyNode(Node):
    def __init__(self):
        node_name="minimal_sub"
        super().__init__(node_name)
        self.__sub = self.create_subscription(String, TOPIC, self.__sub_handler, 10)
        self.__sub
        self.get_logger().info("start minimal sub")

    def __sub_handler(self, msg: String):
        self.get_logger().info(msg.data)

def main(args=None):
    rclpy.init(args=args)
    node = MyNode()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

setup.py

# Add entry points
entry_points={
        'console_scripts': [
            "minimal_pub=py_tutorial_pkg.minimal_pub:main",
            "minimal_sub=py_tutorial_pkg.minimal_sub:main"
        ],
    }

first run

terminal1

ros2 run py_tutorial_pkg minimal_pub

terminal2

ros2 run py_tutorial_pkg minimal_sub

terminal3

# nodes
ros2 node list
/minimal_pub
/minimal_sub

# topics
ros2 topic list
/parameter_events
/rosout
/simple
/simple1

# info topic /simple
ros2 topic info /simple
Type: std_msgs/msg/String
Publisher count: 1
Subscription count: 0

# info topic /simple1
ros2 topic info /simple1
Type: std_msgs/msg/String
Publisher count: 0
Subscription count: 1

usage remapping sub topic

terminal2

ros2 run py_tutorial_pkg minimal_sub --ros-args -r simple1:=simple

terminal3

ros2 topic list
/parameter_events
/rosout
/simple

launch with remapping

run_minimal_1.launch.py

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    ld = LaunchDescription()

    pub_node =  Node(
            package='py_tutorial_pkg',
            executable='minimal_pub'
        )

    sub_node =  Node(
            package='py_tutorial_pkg',
            executable='minimal_sub',
            remappings=[
                ('/simple1', '/simple'),
            ]
        )

    ld.add_action(pub_node)
    ld.add_action(sub_node)
    return ld

terminal1

ros2 launch py_tutorial_pkg run_minimal1.launch.py

terminal2

# nodes
ros2 node list
/minimal_pub
/minimal_sub

# topics
ros2 topic list
/parameter_events
/rosout
/simple

---

Add namespace

terminal1

ros2 run py_tutorial_pkg minimal_pub --ros-args -r __ns:=/demo

terminal2

ros2 run py_tutorial_pkg minimal_sub --ros-args -r __ns:=/other_demo

terminal3

# nodes
ros2 node list
/demo/minimal_pub
/other_demo/minimal_sub

# topics
ros2 topic list
/demo/simple
/other_demo/simple1

remap pub this time

terminal1

ros2 run py_tutorial_pkg minimal_pub --ros-args -r __ns:=/demo -r /demo/simple:=/other_demo/simple1

terminal3

# nodes
ros2 topic list
/demo/other_demo/simple1
/other_demo/simple1

# topics
ros2 topic list
/other_demo/simple1

with launch file

• Add namespace
• Remap topic with full namespace

run_minimal_2.launch.py

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    ld = LaunchDescription()

    pub_node =  Node(
            package='py_tutorial_pkg',
            executable='minimal_pub',
            namespace="/demo",
            remappings=[
                ('/demo/simple', '/other_demo/simple1'),
            ]
        )

    sub_node =  Node(
            package='py_tutorial_pkg',
            executable='minimal_sub',
            namespace="/other_demo"

        )

    ld.add_action(pub_node)
    ld.add_action(sub_node)
    return ld